I ran into the same problem with the 70-tonner; the trucks barely pivoted, and the loco relied mainly on its tiny wheelbase to get through curves. What I did was to slightly trim the front and back edges of the tabs on the trucks that snap them to the frame. I'm not cool enough to post images yet, but you can see the tabs in the first picture in Pete's post above. Making the tabs narrower lets the truck pivot a little further in its slot in the frame.
I figured it's the pivot point that's the issue. I wasn't sure whether it was best to go after the gear tower on the trucks, or to nibble out more frame. Obviously the trucks are easier to get to... I may give that a try.
Thank you. You've got my vote for joining the Cool Kids!
Lee